This invention relates to a method for forming a desired resin pattern which comprises selectively polymerizing a photopolymerizable resin in the necessary portion of a layer composed of the photopolymerizable resin, a radical generator and a sensitizing agent by utilizing a light radiated from a photosensitive substance to form a desired resin pattern on said layer. The method of this invention is particularly effective in fields requiring a high resolution including a field of the production of a printed wiring board (PWB) of a narrow pad pitch used in surface mount or a printed wiring board having circuits of a narrow pattern pitch, and the like.
It has heretofore been difficult that in a substrate having a narrow pitch conductor layers or the like, the space between the conductor layers is exactly filled with an insulating substance for protecting a circuit such as a solder resist.
For example, in the field of a printed wiring board, the surface mounting of devices is allowed to proceed by the high densification or high integration of LSI and IC. In the case of the surface mount, unlike a conventional throughhole mount in which a plating is applied to the inner wall of a throughhole, pads which are bonding portions of lead pins of an LSI package are formed on the surface of a wiring board, a solder layer is provided on each of the pads and the lead pins are bonded to the pads using this solder layer as an adhesive.
Until before several years, there had been mainly used the QFP (Quad Flat Package) type packages in which the number of pins is about 100 to 200. In this case, the lead pin pitch is about 0.65 to 0.5 mm and it is possible to form solder layers by a conventional screen printing method, by a solder coat method or the like. In the case of the screen printing method, a creamy solder is directly coated on a pad for connecting the pad with a pin. In addition, in the case of the solder coat method, a PWB is immersed in a molten solder and thereafter pulled up and a high-temperature air is blown thereagainst to blow off the solder, thereby forming a solder layer on the pad.
When the pad pitch is not less than 0.5 mm as mentioned above, the pad portion on which the package can be mounted can be obtained.
However, high densification and high integration of LSI have proceeded more and more and there has now appeared such a package as TCP for loading TAB having about 500 to 600 pins.
In packages having as many pins as mentioned above, the pin pitch becomes 0.2 to 0.3 mm, and when a screen printing method is used a position deviation is necessarily caused, and when a solder coat method is used the solder between pads is not completely removed. Both of them cause a short circuit.
No technique for printed wiring boards capable of responding to the mounting of a package having as narrow a pin pitch as about 0.2 to 0.3 mm has been settled by now.
Among various techniques, a super solder method and an electroless solder plating method have been tried as methods for forming a solder layer on a pad.
In the super solder method, creamy solder is coated on the whole of a pad area and the solder is heated by a reflow soldering, upon which the melted solder gathers on the copper portion of the pad by surface tension to form a solder layer. However, this has such a problem that unless the heating is uniformly effected, a variation is caused in the thickness of the solder layer between pads.
In the case of the electroless solder plating method, a solder plate layer is formed in such a matter as to be permuted for a part of the copper layer, so that this is disadvantageous in that the thickness of the copper layer in the pad portion becomes thinner than the necessary thickness. For preventing this, it has been considered that the thickness of the first copper panel plate is designed larger than the necessary thickness. However, making the thickness of the copper plate larger than the necessary thickness is not suitable for the formation of a fine pattern.
However, even the above-mentioned super solder method and screen printing method are considered to become applicable to such a narrow pitch article (for example, wiring board) if the narrow space between pads can be filled with an insulating layer.
That is to say, in the case of the super solder method, creamy solder is applied to the whole of the irregular surface of the pad area and heated and melted, upon which a solder layer is formed on the pad portion owing to the surface tension of the molten solder and the affinity of the molten solder to copper in the pad portion. When a super solder method is applied to a wiring board in which the space between pads is filled with an insulator so that the pad portion and the insulator layer form substantially a plane, it is considered that the variation of thickness of the coated creamy solder layer in each position becomes small and the variation of thickness of solder layer formed on each pad due to uneven heating becomes small.
In the case of the screen printing method, when the pad pitch becomes narrow, the printing position of the creamy solder tends to be deviated and in this case, the creamy solder in the deviated portion falls down in the space between pads and tends to form a bridge between the pad and the adjacent pad when heated and melted. However, when the screen printing method is applied to a wiring board in which the space between pads is filled with an insulator to make the circumference of the pad portion close to a plane, it is considered that even if a creamy solder should be printed in a deviated position it would remain in that position without falling down to a lower position between the pads when it is heated and melted or it would shift in the direction to the copper layer of the pad owing to the surface tension of the molten solder and the affinity of the solder to copper.
The filling of the space between pads with an insulator is considered to be made possible by coating a photosensitive resin on the pads, exposing only the space portions between the pads through a photomask to a light to polymerize only the exposed resin and removing the unpolymerized resin by development. However, when the pad pitch is 0.2 mm and the width of the pad is 0.1 mm, the space between the pads also becomes 0.1 mm, and hence, such a disadvantage is caused that when the position of the photomask is deviated the photosensitive resin layer overlays the pad as much as the position deviation. Accordingly, the precision of the alignment of a photomask becomes required to be .+-.0, and it is very difficult to completely fill only the space between pads with an insulator layer.
When the substrate size is 500.times.500 mm and QFP pads (0.5-mm pitch) are present at both ends, even if one of the QFP pads and a negative film are aligned without a bit difference and are placed in the ideal state, the shrinkage of the substrate by 0.02% makes the substrate size shorter than the negative film and causes a deviation of 100 .mu.m of the film at the other end QFP pad. As a result, the solder resist comes to run on the pad and consequently a failure due to fog is caused. Therefore, a substrate maker has been strongly asked to provide a substrate having a good dimensional stability (a substrate whose elongation and shrinkage are small or a substrate whose variation in elongation and shrinkage is small). Experientially, it is general that the dimension of a substrate material is varied by about 0.02 to 0.04% in the course of production of a PWB, and a corrective film is planned supposing this variation in the preparation of the above-mentioned negative film. However, the variation in dimensional change is great every sheet of substrate or every number of article different in the remaining area of pattern copper and usually, the dimensional change is varied exactly by +0.01-0.03%. Therefore, ideally, a method of forming a resist pattern without using a negative film is desired.